Extrusion processes are used in a variety of industries to form a wide range of products. One type of extrusion process uses a ceramic-forming material that forms a plastic mix or ‘batch material” that is extruded through a die orifice to form a shaped article.
More recently, ceramic honeycomb-shaped articles having a multitude of cells or passages separated by thin walls running parallel to the longitudinal axis of the structure have been formed via extrusion and used as filters (“ceramic filters”) for a variety of applications, including particulate filters for combustion engines.
A number of parameters need to be controlled in the extrusion process for the desired article to maintain its post-extrusion form and to ultimately form an article that meets is particular design and/or performance requirements. Such parameters include, for example, the particular composition of the mix that makes up the batch material, and the moisture content of extruded “logs” that can subsequently be dried (a “dried log”) and fired to form a ceramic article A batch material having insufficient moisture will not extrude properly and could lead to the formation of cracks (including invisible microcracks) in the final article. On the other hand, an organic batch material having too much moisture will also not extrude properly and could lead to deformation (e.g., sagging) of the extruded article.
The moisture content of a dried log needs to be measured to determine whether it meets the dryness specification, which is typically on the order of 1% or so, prior to the dried log being cut and fired. A log having too much moisture will be damaged (e.g., “smeared”) upon cutting, and can also damage the saw blade used to cut the log.
It is thus desirable to be able to measure the moisture content of ceramic-forming material used to make the ceramic articles such as ceramic filters. One moisture-content measurement technique is called “loss on drying” (LOD), which is a destructive, off-line technique that can take hours to complete. Other moisture-content measurement techniques that rely on pyrometers to measure log temperature tend to be inaccurate because they measure surface temperature, which is often significantly different from the bulk temperature (and thus the water content) of the ceramic material.
Consequently, what is needed are efficient, in-line methods and systems for measuring moisture content of the ceramic material during the ceramic article manufacturing process.